Integrated circuits (ICs) often require selectively permanently programmable electrical connections between circuit nodes. Such a connection can be implemented by an antifuse link (antifuse). Antifuses are fabricated with structure similar to that of a capacitor; two conductive electrical terminals are separated by a dielectric layer. An unprogrammed "off" state, in which the antifuse is fabricated, presents a high resistance between the antifuse terminals. The antifuse can also be programmed to an "on" state where a low resistance connection between the antifuse terminals is desired. To program an antifuse "on," a large programming voltage is applied across the antifuse terminals, breaking down the interposed dielectric and forming a conductive link between the antifuse terminals. However, the conductive link between the antifuse terminals is sometimes only marginally conductive, leaving a relatively high resistance of up to approximately 400 K.OMEGA. through the antifuse in its marginally "on" state. An even larger voltage could be used to better break down the dielectric, but such a voltage is more likely to cause damage to other circuitry.
Antifuses are often used to permanently store binary data on an IC. Binary logic states are represented by the "on" and "off" states of the antifuse. An antifuse detect circuit may be used to read the state of the antifuse. A typical antifuse detect circuit forms a resistor divider with the antifuse resistance during reading the state of the antifuse element. A marginally "on" antifuse presents a relatively high resistance, which is not well controlled. The resistor divider may only provide a difference on the order of millivolts between a marginally "on" and an "off" antifuse, which is difficult to consistently accurately detect. Thus, a marginally "on" antifuse may erroneously be detected as "off" if the antifuse resistance is sufficiently large.
There is a need in the art to provide a robust antifuse detect circuit for accurate reading of the state of an antifuse, including an antifuse having an only marginally conductive "on" state. For example, antifuses are often used in memory cell arrays such as dynamic random access memories (DRAMs). After testing the DRAM for failing memory cells, failing cell addresses in a DRAM may be remapped to functional cell addresses by selective permanent programming of antifuse elements. This remapping is typically done on a row or column basis. In operating the DRAM, the state of the antifuse must be determined in a timely manner such that memory cells are quickly addressed. A timely and robust antifuse detect circuit is needed for memory cell arrays and for a wide variety of other applications in which antifuse elements may be used.